Chemical mechanical polishing (CMP—also referred to as chemical mechanical planarization or chemical mechanical etching) is a well-known technology used in fabricating advanced photonic, microelectromechanical (MEM), and microelectronic devices, such as semiconductor wafers. See e.g. Chemical-Mechanical Processing (Springer Series in Materials Science), Michael R. Oliver, Springer Publ., (Mar. 24, 2006); Microchip Fabrication, Peter Van Zant, McGraw-Hill (2004); Chemical Mechanical Polishing in Silicon Processing, Volume 63 (Semiconductors and Semimetals), Eds. Shin Hwa Li and Robert O. Miller, Academic Press (1999); Chemical Mechanical Planarization of Microelectronic Materials, Steigerwald et al., John Wiley & Sons (1997).
In the field of semiconductor fabrication, chemical mechanical polishing is used to planarize metal and/or oxide substrates. CMP uses both chemical and mechanical action to obtain the desired planarity of the surface to be polished. Chemical action is provided by a mixture of chemicals which is termed as “slurry” usually consisting of abrasives and various additive compounds which include family of chelating agents, passivating agents, oxidizing agents, accelerators etc., Mechanical action is provided by pressing to-be-polished substrate onto the surface of a polishing pad adhered to a moving platen. Movement of platen is typically linear, rotational or orbital.
In a typical chemical mechanical polishing process, a rotating wafer holder brings the wafer to be in contact with a polishing pad or CMP pad. One of the key consumables in conventional CMP processes is the CMP pad or polishing pad. The CMP pad is mounted on a rotating platen. A polishing medium, such as an abrasive slurry, is applied between the wafer and the pad.
Suitable polishing pads made of polymer which include, but not limited to polyvinyl chloride, polyvinyl fluoride, nylon, fluorocarbon, polycarbonate, polyester, polyacrylate, polyether, polyethylene, polyamide, polyurethane, polystyrene, polypropylene, conformed products thereof, and mixtures thereof.
The useable lifetime of the polishing pad is typically determined by observing detrimental decrease in the removal rate (RR), increase in non-uniformity, and/or an increase in surface defects. The deterioration of the pad performance can be traced to a wide range of physical changes in pad such as significant reduction in pad groove depth, porosity, modulus strength, etc. The sources of these physical changes are usually attributed to mechanical stress such as polishing down force, conditioning, temperature, etc.
In addition, the performance decrease over the course of time of polishing can be attributed to an effect called glazing. The pad glazing is often a result of pad debris covering the asperities and pores of the pad thereby leading to a lower material removal rate. These aspects of the physical change have been well documented and studied. See for example, Chen et al., Operational aspects of chemical mechanical polishing, Journal of The Electrochemical Society, 147 (10) 3922-3930 (2000); Ul-hasan et al., Spectroscopic and topographic investigations of nanoparticle abrasive retention in polyurethane CMP pads for Cu CMP, Mater. Res. Soc. Symp. Proc. Vol. 991 (2007); Tregub et al., Pad degradation during CMP process: Effect of soak in slurry and water on thermal and mechanical properties of the CMP pads, Mater. Res. Soc. Symp. Proc. Vol. 767 (2003); Moy et al., Polyurethane pad degradation and wear due to tungsten and oxide CMP, Mater. Res. Soc. Symp. Proc. Vol. 671 (2001).
Other factors that could significantly alter the physical changes of the pad include chemical reactions such as hydrolysis and oxidation. There has been much less documentation in the art which addresses this aspect of changes in pad lifetime.
Specifically, a CMP slurry can interact with the polishing pad surface and cause chemical reactions within. For example, under extreme acidic and alkaline conditions, —COC— and —COOC— groups in polyurethane elastomers can hydrolyze over time at an accelerated rate. A chemical change in pad materials may lead to a significant alteration of pad surface properties such as surface tension or hydrophibicity, which in turn may lead to enhanced or suppressed removal rates based on its surface property of the pad. A chemical alteration at this level cannot be easily remediated by physical means such as pad conditioning.
As such, a problem remains in the art with respect to maintaining sufficient lifetime for polishing pads and also for maintaining adequate removal rates and selectivity during the lifetime of the polishing pad.
Surprisingly, these problems in the art can be overcome with the addition of environmentally benign agents that provide a buffer mechanism to reduce the impact of the chemical attacks on the polishing pads. More specifically, as disclosed by this invention, a set of hydrophobic amino acids is found to be effective in reducing the effect of chemical attack by strong acids on polishing pads.